Disclosed is a method for determining the accumulation rate of gases in an enclosed volume.
An important area of commerce is the packaging of goods subject to deterioration, such as foods, cosmetics, medicines, and the like. Deterioration often results from the diffusion of gases through the packaging material. This diffusion may be in the form of the loss of carbonation in a beverage, the loss of fragrance in a cosmetic, or the accumulation of oxygen or other reactive permeants into the package causing the contents thereof to undergo undesired oxidation or other reaction. Goods packaged in plastics are particularly susceptible to diffusion-related deterioration.
Widespread research has been undertaken to identify methods for reducing the deterioration of goods packaged in plastics by interfering with the diffusive transport of various permeants through the packaging material. In order to make those advances, it is necessary to have quick, accurate, and reproducible methods for characterizing the diffusive processes which contribute to product deterioration. One particularly difficult problem is presented when the permeant of interest is present at high concentrations within the packaging material to be characterized.
This is the situation, for example, when the infusion of oxygen into a plastic package is to be characterized. Oxygen exhibits quite high solubility in many plastics used in the packaging industry. Because oxygen is present in the ambient atmosphere, any such oxygen-soluble material in commercial use will be suffused with oxygen. In such a case, the infusion rate of oxygen into an initially oxygen-free package will be governed both by the rate of permeation of atmospheric oxygen through the packaging material, and by the rate of oxygen desorption from the packaging material. Any effort to control oxygen infusion into the package must take account of both processes, and it is therefore necessary to characterize each process independently.
The present invention provides a method for the separate, simultaneous characterization of desorption and permeation using a single specimen.
Representative of the methods most commonly employed in the art for measuring permeation rates is that for determining the oxygen permeation rate of plastics as described in ASTM D3958. In this procedure one side of a film or container is subject to a constant partial pressure of oxygen while the other side of the film or container is swept with a continuous stream of nitrogen. Oxygen diffusing into the nitrogen stream is detected by some quantitative method to give the rate of oxygen infusion. The Ox-Tran series of test stands manufactured by Modem Controls, Inc., Minneapolis, Minn., is commonly employed for oxygen permeability measurements; the detector is a fuel cell.
When the test specimen is suffused with oxygen, there is no known practical method for separating the oxygen desorption contribution to the total rate of oxygen accumulation from the oxygen permeation contribution. The problem is greatly aggravated when the sample to be characterized is in some complex shape such as that of a bottle where the introduction of inhomogeneities in thickness and crystallinity may be inherent in the process of formation. In the current state of the art, the permeation contribution is determined simply by waiting to take data until the desorption contribution becomes negligible. In other words, meaningful permeation data cannot be taken until the specimen has been outgassed. That point is usually determined to be the point at which the rate of permeant accumulation in the previously permeant free volume becomes linear with time. In certain circumstances of current interest, this may take as long as several weeks.
The desorption component may be determined by subjecting a specimen to vacuum or an inert atmosphere on both sides and determining the rate of degassing. That is to say, it must be performed with the exclusion of any atmospheric permeant contribution.
In a different approach, Hanke, East German Patent 119472, determines the lag time for oxygen diffusion in construction materials by sealing a specimen between two chambers which are maintained at equal oxygen partial pressures, introducing a rare isotopic species on one side of the specimen, and observing the time required to detect the species on the other side. 18O2, 15N2, and 85Kr, which is radioactive, are all isotopic species employed in Hanke. While Hanke""s method is suitable for analyzing permeation through the specimen, it is completely unsuitable for the simultaneous determination of the permation and desorption rates. In Hanke""s case, oxygen desorption was neither of concern, nor was it likely to be of significance.
The present invention provides a method for the simultaneous determination of permeation rate through and desorption rate from a polymeric specimen, the method comprising:
sealingly separating a first volume from a second volume with a specimen to be tested, said specimen being suffused with a first isotope of a gas or vapor;
introducing in said first volume a second isotope of said gas or vapor, said second isotope being detectably distinguishable from said first isotope;
in said second volume, adjusting the partial pressure of both said isotopes of said gas or vapor to a negligible value compared to that of said second isotope of said gas or vapor in said first volume;
and, distinguishably detecting the concentration of each said diffusing isotope of said permeant in said second volume.
Further provided in the present invention is an apparatus fluid seal adapted for use in testing gas permeability of plastic bottles, the apparatus comprising:
a cylinder having an interior wall, a first end, and a second end, the ends being open;
a plug having a surface said plug disposed in said first end of said cylinder in such manner that a gap exists between the surface of the plug and the interior wall of the cylinder;
a first seal disposed within said gap proximate to said first end, and a second seal disposed within said gap proximate to said second end, said seals, said wall, and said plug surface defining a volume; and,
a means provided within said plug permitting introduction and removal of fluid within said defined volume.